The antiviral characteristics of monoclonal antibody IAM-41-2F5 (2F5) were determined in cell culture. The antibody had been previously shown to bind a specific sequence, ELDKWA, within the external domain of the gp41 envelope glycoprotein human immunodeficiency virus type 1 (HIV-1). Selection by 2F5 of recombinant phage from an epitope library confirmed the identification of the antibody's binding determinant. The antibody was found to be capable of neutralizing a broad range of lymphoid cell culture-adapted HIV-1 variants as well as HIV-1 primary isolates. Sequence analysis of the latter showed that neutralization was related to the presence of the antibody binding site. From kinetic measurements using an epitope-containing peptide or gp41, the half-time of dissociation for 2F5 was determined to be 122 min for the peptide and 156 min for gp41. The region of gp41 expressing this sequence exhibits greater conservation among HIV-1 isolates than do the variable domains of gp120.